1. Field of the Invention
The present invention is related to an optical element module used in optical fiber communication. In particular, the present invention is related to an optical element module which is durable against environmental temperature fluctuations.
2. Description of the Related Art
In order to connect optical fibers to an optical element of an optical waveguide or the like, an optical element module is used in which the optical element is provided inside a casing, and the optical fibers are introduced from the outside of the casing and connected to the optical element inside the casing. The structure of a related art optical element module is shown in FIG. 1.
In FIG. 1, 50 represents an optical element module, 51 represents a casing, 52 represents an optical waveguide protecting plate which covers an optical waveguide, 53a and 53b represent optical fiber holding portions, 54 represents a coated optical fiber, 55 represents a bare optical fiber, 56 represents taped optical fibers, 57 represents bare optical fibers, and 58 and 59 represent adhesive fixing portions.
The types of optical fibers described in this application are shown in FIGS. 2A,2B,2C and FIG. 3. In FIGS. 2A,2B and 2C, 30a represents an optical fiber core portion, 30b represents an optical fiber clad portion, 31 represents a primary coating, and 32 represents a secondary coating. The optical fiber (see FIG. 2C) which is coated up to the secondary coating is called a coated optical fiber, the optical fiber (see FIG. 2B) which is coated up to the primary coating is called a primary coated optical fiber, and the optical fiber (see FIG. 2A) which does not have even the primary coating is called a bare optical fiber. Normally, the external diameters of the coated optical fiber, the primary coated optical fiber and the bare optical fiber are 900 μm, 250 μm and 125 μm, respectively. In FIG. 3, 30a represents an optical fiber core portion, 30b represents an optical fiber cladding portion, 31 represents a primary coating, and 33 represents a secondary coating for creating a taped fiber generally called as a fiber ribbon. In the taped optical fiber, the optical fiber which is coated up to the secondary coating is called a taped optical fiber, the optical fiber which is coated up to the primary coating is called a primary coated optical fiber, and the optical fiber which does not have even the primary coating is called a bare optical fiber.
The optical element module 50 shown in FIG. 1 includes one light input and four light outputs, wherein the input light from one optical fiber is divided into four branches by an optical waveguide formed by four branching optical circuits, and the branched light is optically outputted to four optical fibers. The optical fibers are connected to the input and output end surfaces of the optical waveguide, and the optical waveguide and the like are provided inside the casing. The coated optical fiber 54 for light input is guided from the outside into the inside of the casing 51, and the primary coating and the secondary coating are removed to form the bare optical fiber 55. The bare optical fiber 55 is housed in the optical fiber holding portion 53a which includes a V-groove or the like, and is fixed to the optical waveguide in a state where the optical axis is aligned with the input portion of the optical waveguide (not shown in the drawings). The taped optical fibers 56 for light output are guided from the outside into the inside of the casing 51, and the primary coating and the secondary coating are removed to form the bare optical fibers 57. The bare optical fibers 57 are housed in the optical fiber holding portion 53b which includes V-grooves or the like, and are fixed to the optical waveguide in a state where the optical axes are aligned with the output portion of the optical waveguide (not shown in the drawings).
The optical waveguide protecting plate 52 is fixed to the top of the optical waveguide, and by increasing the contact surface area of the optical fiber holding portions 53a and 53b, a stable connection between the bare optical fibers 55, 57 and the optical waveguide is maintained. The optical waveguide is fixed to a bottom portion of the casing 51 by an adhesive. The coated optical fiber 54 for light input is fixed to the casing 51 by the adhesive fixing portion 58, and the taped optical fibers 56 for light output are fixed to the casing 51 by the adhesive fixing portion 59.
In accordance with the structure of an optical access line network suited to the PON (Passive Optical Network) topology of recent years, there is a demand to provide an optical element module internally equipped with a waveguide in an outdoor facility, and this creates the need to make the optical element module adapted to outdoor use. In particular, because optical fibers are housed in a narrow outdoor facility, primary coated optical fibers need to be used as optical fibers for light input and light output.
In the related art optical element module, in the case where the optical fibers for light input and light output are made primary coated optical fibers, it was determined that stable characteristics can not be obtained in an environment where there are severe temperature changes. A temperature cycle test was carried out for the case where the optical fiber for light input in the related art optical element module was made a primary coated optical fiber. The test results are shown in FIG. 4. In FIG. 4, the horizontal axis represents the test time, and the vertical axis of the graphs in the upper portion of FIG. 4 represents the increased optical loss of the optical element module at each temperature. Eight hours formed one cycle, and from the results obtained by carrying out a temperature cycle test from −40° C. to +75° C., it was understood that there are large fluctuations of the optical loss of the optical element module when the environmental temperature of the optical element module changes.
As for the cause of the large fluctuations of the optical loss shown in FIG. 4, in the case where the environmental temperature changes, in particular, in the case where the environmental temperature reaches a low temperature, because the primary coated optical fibers and the casing of the optical element module contract, a tensile and compressive stress is generated between the two points of the optical fiber holding portion which adhesively fixes the primary coated optical fiber for light input and the adhesive fixing portion which fixes such optical fiber to the casing by an adhesive, and as a result, micro bends are created in the primary coated optical fiber for light input, and this was understood to cause an increase in the optical loss.
Further, when coated optical fibers like that shown in FIG. 2C are made the optical fibers for light input and light output, slippage occurs between the primary coating and the secondary coating fixed by an adhesive, and because the stress on the primary coating is reduced, the optical loss does not increase even when the environmental temperature reaches a low temperature. This is the same for the taped optical fiber of FIG. 3. Even when the outside of the secondary coating of the taped optical fiber is fixed by an adhesive, slippage occurs between the primary coating and the secondary coating, and because the stress on the primary coating is reduced, the optical loss does not increase even when the environmental temperature reaches a low temperature.